1. Field of the Invention
This invention relates to an ultrasonic diagnostic apparatus and, more particularly, to an ultrasonic diagnostic apparatus for displaying a static or so-called "frozen" image or picture by means of a picture signal which includes picture element data representing each of a plurality of picture elements contained in a single frame.
2. Description of the Prior Art
Ultrasonic diagnostic equipment of the above-described type is in wide use. A characteristic of a real-time ultrasonic diagnostic apparatus is that portions of the human body such as the human heart, or a fetus while still within the mother's womb, can be observed on a real-time basis. Such an apparatus capable of displaying a dynamic image of this kind is equipped with a so-called "freeze" function which allows the operator or attendant to freeze the dynamic image at will in order to obtain instantaneous bioinformation at a given point in time. A frozen image, produced by direct freezing of the dynamic image at a certain time, generally gives a picture quality that is poorer than that of the dynamic image. Ordinarily, with a standard television system, a picture is displayed on the display screen at a speed of 30 frames per second. When freezing a dynamic image of this kind, the usual method comprises temporarily storing, in memory, picture element data representing each of the picture elements contained in one frame at a given point in time, and repeately reading the items of data out of the memory to reconstruct them as a static picture of one frame. Accordingly, though there is some difference depending upon visual quality, the quantity of information obtained at the rate of 30 frames per second generally is greater than that obtained in the form of a frozen image from one frame of a dynamic image. With a dynamic image, therefore, visual discrimination of the boundary regions of an image such a tomograph produced in ultrasonic diagnosis is facilitated owing to movement of the boundary regions. Moreover, with a frozen image, a random noise component also is frozen into the image and has a pronounced influence on visual clarity.
Methods have been proposed for improving upon the picture quality of a frozen image obtained by freezing a dynamic image. According to one such method, picture signals obtained by scanning the same portion of the specimen a plurality of times by ultrasonic waves are stored in memories, referred to as frame memories, provided for corresponding frames. Then, for reconstruction, the stored items of data are read out of each of these memories and added together, thereby improving the signal-to-noise ratio. The ultrasonic diagnostic apparatus for realizing this method has been disclosed in Japanese Patent Application Laid-Open No. 55-45422. The apparatus is complex in construction and high in cost, however, owing to the need for a number of frame memories equivalent to the number of frames that are to take part in the addition operation.
Another proposal relates to an ultrasonic video apparatus, disclosed in Japanese Patent Application Laid-Open No. 55-145489. According to this apparatus, video from the same target area is sampled continuously for a plurality of frames, and a serial picture having a differential noise component is obtained, serially in terms of time, from a serial picture input and a serial picture output of an identical region, said serial picture output having already been made to approach a target value. The weighted average of the resulting serial picture is then integrated, whereby the serial picture output is constantly made to approach a reproducible target value to improve upon the signal-to-noise ratio. With this apparatus, a division operation is carried out to reduce the weighted average of the serial picture containing noise, with the result that several bits of lower order digits are dropped each time a division is performed. The end result is a deterioration in the accuracy with which the serial picture output can be made to approach on the target value.
Still another proposal, disclosed in Japanese Patent Application Laid-Open No. 56-21079, relates to an ultrasonic receiver apparatus for integrating picture signals by using analog elements. With analog elements, however, the number of effective digits is not as large as with digital elements, so that no improvement in picture signal accuracy can be achieved. Such apparatus is therefore suited to limited applications not adaptable to digital processing.